Mercury Arc Rectifier

Slightly off topic but in the spirit of this thread. I went to the Kempton engines this morning and went to take a thermal image of the MARs, they are 'protected' by some perspex which doesn't let short wave IR (infra red) through, no picture. I had a chat with one of the people there and indeed the two MARs supply the 200V DC to the plant, the DC is raw six phase rectified. It was supplying about 5 amps.

But I did take one of the huge engine running in IR.

[Top Cap]

I saw you taking that shot but did not know it was you
Glad to see those MARs working.

[IanBland]

When I was at the Adelphi in the early 1980s (Me And My Girl, specifically), the mercury arc rectifiers ("mekons") were still in use. An awesome sight. I also with a colleague dismantled a large collection of DC gear in the dimmer room which included various huge finned rectifier diodes, but the specific purpose of the gear I cannot recall.

There had at one time (before my time) been a DC main in the West End feeding the theatres, and when that was decommissioned a lot of DC gear was installed to feed carbon arc follow spots etc, I think it would have been the 1960s.

[Top Cap]

My mate who was with me at Kempton did a short video of the mekons and placed it on You Tube for all to see.
https://www.youtube.com/watch?v=XYZY...ature=youtu.be

[M0FYA Andy]

Nicely filmed, but I think you should suggest that he removes the 'grocer's apostrophe' from the title page.............

Andy

[Dai Corner]

Members might be interested to learn that MARs used to travel at 100mph up and down the West Coast Main line between London, Birmingham, Manchester, Liverpool and Glasgow in the early AC electric locomotives.

http://www.aclocogroup.co.uk/stock81002.php

[avocollector]

is my memory faulty or did mercury arc rectifiers appear in early battery chargers? I recall my father in law using something with a large globe inside that gave off a blue light and buzzed frantically to charge the batteries (standard 12 volt car ones) on his mother's ancient (even in the early 70's) electric wheelchair. The fact that H2 was happily bubbling out of these batteries I realise in retrospect might be the reason why the chair never went very far!!.

[Lucien Nunes]

Mercury Arc recs have been extensively used for charging large batteries, however the small unit you mention is likely to have been a mercury vapour rectifier with a heated cathode. Placing a small amount of liquid mercury in the envelope of the thermionic valve rectifier provides a source of vapour to be ionised and neutralise the space charge, allowing the valve to rectify currents up to the total emission current with only 15-20V forward drop. An MAR however also benefits from near-infinite cathode life and much greater possible emission current available from a hotspot on a pool of liquid, at the expense of requiring starting and sustaining gear and usually requiring a rigidly fixed installation of the bulb.

For a medium-size MAR batttery charger see Chloride / Nevelin charging MAR

[avocollector]

Thanks for the info Lucien - if I recall correctly it was housed in a large red metal box (at least a foot each side) with slots in it through which you could see the transformer and bulb, the glow, and hear the buzzing. Pity I never looked for it when my Father in law passed away.

[kalee20]

So were MAR's used on electric trains? I've always thought trains ran on DC, with any rectification taking place at the trackside. Quite apart from possible mercury sloshing about in a high-speed locomotive, AC return through an iron rail... just think of the skin effect loss!

[Hartley118]

AFAIK, apart from tube trains, practically all mainline electric trains these days run on AC, but with onboard solid state rectification and no sloshing mercury!

Martin

[high_vacuum_house]

Some of the early 1960's AC electric locomotives had ignitrons in which are a form of mercury arc rectifier.
Solid state rectifiers soon took over though.

The bulbs were steel rather than glass for robustness with ceramic seals and produced DC for the traction motors as AC motors and control equipment at the time was not good enough for high torque slow speed situations (i.e. starting a train moving)

Christopher Capener

[julie_m]

The old Southern Electric Railway (which later became Network South East; as far North as Bedford and as far West as Reading) used (well, still uses) 750 V DC from a third rail. The modern system of 25kV overhead wires was a joint B.R. / S.N.C.F. invention, relying on very big silicon diodes and thyristors and forced-air cooling. But there are plenty of other sites on the Internet for train talk; so to get this back on the rails (O.K., last mention) I will add that Mercury Arc Rectifiers were indeed used to supply power to the S.E.R. But these were static, lineside installations, not on board moving trains.

[DangerMan]

The first generation (AL1-AL4) 25kV electric loco's and EMU's for the west coast main line used steel tank mercury arc rectifiers of various types and suffered quite a few back-fires and loss of vacuum due to vibration, much to the distress of the transformers and the rectifiers themselves. The later AL5 incarnations used germanium devices from the outset, then silicon for some AL5's, and AL6 onwards.
All the surviving early loco's and stock did get updated to silicon rectifiers though.

[julie_m]

Well, that's interesting. I never knew they dared to use MARs on trains.

Doesn't mercury undergo a reaction with other metals? Something is flickering dimly in my mind about thermometers not being allowed on aeroplanes .....

[DangerMan]

Originally Posted by julie_m

Doesn't mercury undergo a reaction with other metals? Something is flickering dimly in my mind about thermometers not being allowed on aeroplanes .....

At a first guess, does aluminium (or other constituent parts of airframe aluminium alloy) dissolve in mercury if there is a spillage? That could be nasty!

[kalee20]

Yes, mercury forms an amalgam with aluminium, dissolving it. Then the dissolved aluminium atoms at the surface of the liquid oxidise in air, like any bare aluminium does. But the oxide can't stick to the surface of a liquid like it can on solid aluminium, so it flakes off. And then the mercury goes on to dissolve a bit more aluminium. Result: a single drop of mercury can wreck an indefinite amount of aluminium structure.

[feathercurls]

Originally Posted by avocollector

is my memory faulty or did mercury arc rectifiers appear in early battery chargers? I recall my father in law using something with a large globe inside that gave off a blue light and buzzed frantically to charge the batteries (standard 12 volt car ones) on his mother's ancient (even in the early 70's) electric wheelchair.

Maybe a Tungar bulb charger?
https://www.youtube.com/watch?v=o_fc8mwdoe4

[KeithsTV]

I remember visiting my mothers uncle, who owned a garage in the Cotswolds and had an ancient battery charger at the back of the workshop. I can't remember if it had a mercury arc rectifier but he was amazed that the rectifier fitted could be replaced by a small silicon rectifier.

Keith

[Synchrodyne]

Originally Posted by julie_m

Well, that's interesting. I never knew they dared to use MARs on trains.

The major work in adapting industrial ignitron-type single-anode mercury-arc rectifiers for traction applications was done by Westinghouse in the late 1940s. Significant early ignitron users (mid-1950s) in “production” quantities were SNCF (France) and JNR (Japan). Both of these also used the excitron-type single-anode rectifiers. In the SNCF case, some excitron installations were equipped also to act as inverters to facilitate regenerative braking. There were some experimental installations of multi-anode pumpless mercury-arc rectifiers, but probably these simply confirmed that Westinghouse had been on the right track in opting for the single-anode approach. As already noted, the advent of silicon rectifiers suitable for traction purposes foreclosed the traction mercury arc era quite early on, say by 1964. Although I understand that SNCF continued to acquire excitron-fitted locomotives for a while after that because of their regenerative capability. But until the silicon era, the ignitron (or excitron)/DC motor combination was seen as superior to the single-phase motor approach for 25 and 50 Hz installations (and the only real option for 60 Hz), despite the rectifier trials and tribulations, although not so for 16⅔ Hz (for which opinion remained divided even in the silicon era).

So one might say that for a while in the 1950s and into the 1960s, the use of mercury-arc rectifiers for traction purposes was a valid and mainstream approach, and not simply a passing oddity.

Cheers,